Fuel cell systems include stationary ones such as cogeneration fuel cell systems and non-stationary ones for use in portable electronic devices, electric vehicles, etc. Non-stationary fuel cell systems have been proposed recently and direct-type fuel cells, in particular, are receiving attention as ubiquitous mobile power sources that do not need charging, for example, from an AC adapter. Currently, active research and development of direct-type fuel cells is underway.
In direct-oxidation-type fuel cells, a fuel is directly supplied to the anode. The oxidation reaction of the fuel occurs in the anode, while the reduction reaction of oxygen takes place in the cathode. In the case of direct methanol fuel cells using methanol as the fuel, the reaction formulas are as follows.Anode: CH3OH+H2O→CO2+6H++e−  (1)Cathode: 3/2O2+6H++e−→3H2O  (2)
As shown by the formula (1), the anode reaction requires water. When water and a fuel are supplied from outside a fuel cell system, they are stored in a water cartridge and a fuel tank. Thus, additional space is necessary, thereby resulting in a decrease in energy density. It is therefore common in a direct methanol fuel cell to collect and reuse part of the water produced in the cathode (as shown by the formula (2)) in its fuel cell system.
Further, there has been proposed, for example, a circulation-type fuel cell system. In this fuel cell system, not only water but also anode and cathode effluents (which contain unreacted fuel, produced water, etc.) are collected, and the collected effluents are mixed with a high concentration fuel in a fuel tank. The resulting fuel mixture of a predetermined fuel concentration is reused to generate power.
Also, in order to reduce the size and weight of fuel cells and operate them longer, a fuel cell system of “fuel non-circulation (complete consumption)/water circulation (recovery) type” has been proposed (e.g., see Japanese Laid-Open Patent Publication No. 2005-25959 (Document 1)). The basic concept of such fuel cell systems is to bring the supply amount of a fuel of a predetermined concentration as close to the amount of the fuel consumed by power generation as possible, and collect/reuse water without reusing the fuel.
In the fuel cell system of Document 1, in order to prevent harmful substances from being discharged from the fuel cell system, unreacted fuel discharged from the anode is purified with a purifying device or transported to the cathode side, whereby the unreacted fuel is purified inside the power generation section of a fuel cell and purified with a purifying device disposed on the cathode outlet side. However, the catalytic purification inside the power generation section of the fuel cell causes a significant decrease in power generating characteristics.
Besides the above-mentioned proposals, various proposals have been made to prevent harmful substances from being discharged from fuel cell systems. For example, Japanese Laid-Open Patent Publication No. 2005-293974 (Document 2) proposes a fuel cell system of fuel circulation/water circulation type including a gas-liquid separating means and a harmful-substance collecting means (adsorbent such as activated carbon or zeolite), in order to efficiently collect harmful substances produced by power generation and prevent them from being discharged to outside.
Japanese Laid-Open Patent Publication No. 2005-183014 (Document 3) proposes a fuel cell system including a gas-liquid separator that selectively allows gas components in anode and cathode effluents to pass through, and a harmful-substance removal filter with a catalyst that oxidizes the passed gas components for combustion.
However, these related art documents merely propose means for preventing dispersion of harmful substances contained in gas components of effluents from fuel cells and means for purifying the harmful substances in the gas components. These related art documents do not have the concept of catalytically purifying all the effluents containing gas and liquid from the anode and cathode of a fuel cell and collecting water. That is, in related art, gas components discharged from fuel cells are catalytically purified and simply discarded to outside as steam, i.e., they are not effectively reused.
Further, in these related art documents, the liquid collected by the gas-liquid separator undesirably contains fuel. Thus, the fuel concentration in the liquid in the gas-liquid separator varies, thereby resulting in a decrease in power generation stability of the fuel cell system.
It is therefore an object of the present invention to effectively utilize effluents containing gas and liquid from the anode and cathode of a fuel cell and provide a fuel cell system with excellent power generation stability.